Image reading device, image reading method, non-transitory recording medium storing computer readable program, and image formation device

ABSTRACT

It is intended to allow whether or not a noise included in a read image is a noise due to an ink ejection failure to be more reliably determined. An image reading device includes an image reading unit that reads an image formation surface of paper by using a reducing optical system and a control unit. The image reading unit includes a plurality of read sensors disposed at positions displaced from each other in a main scanning direction. The plurality of read sensors are disposed to be able to read the image formation surface by using at least a local region of the image formation surface as a common region to be read. The control unit determines, based on a first image obtained by a first read sensor included in the plurality of read sensors and on a second image obtained by a second read sensor included in the plurality of read sensors, an amount of displacement of a noise resulting from a parallax between the first read sensor and the second read sensor and determines, based on the amount of displacement, whether or not the noise is present on the image formation surface.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present invention claims priority under 35 U.S.C. § 119 to JapanesePatent Application No. 2020-085653, filed on May 15, 2020, the entirecontent of which is incorporated herein by reference.

TECHNOLOGICAL FIELD

The present invention relates to an image reading device, an imagereading method, a non-transitory recording medium storing a computerreadable program, and an image formation device.

BACKGROUND

There has been known an ink-jet image formation device which causes aplurality of nozzles to eject ink to form an image on a sheet. In animage formation device of this type, when an ink ejection failure occursin any of nozzles, an image on a sheet has a missing portioncorresponding to a position of the nozzle with the ejection failure(hereinafter referred to also as an “faulty nozzle”), and the missingportion remains as a missing nozzle defect.

As a technique of preventing degradation of an image quality due to themissing nozzle defect, a technique which specifies a position of themissing nozzle defect and compensates for an image to be formed with thefaulty nozzle based on the specified position of the missing nozzledefect by using a nozzle adjacent to the faulty nozzle or a nozzleserving as a substitute for the faulty nozzle.

Japanese Unexamined Patent Application Publication 2015-58602 (PatentLiterature 1) describes a known technique of determining the presence orabsence of an ink ejection failure, while specifying a faulty nozzleassociated with the ink ejection failure. Japanese Unexamined PatentApplication Publication 2015-58602 (Patent Literature 1) also describesa technique of arranging an image reading unit on a downstream side ofan ink-jet head in a direction of sheet conveyance and when it isdetermined that an image on a sheet read by the image reading unit hasan ink ejection failure, analyzing the image and specifying the faultynozzle.

CITATION LIST

-   [Patent Literature 1] Japanese Unexamined Patent Application    Publication 2015-58602

SUMMARY

The missing nozzle defect remaining in an image on a sheet due to an inkejection failure in a nozzle is a linear image defect (hereinafterreferred to as a “linear defect”) along the direction of sheetconveyance. When the image on the sheet with the linear detect is readby the image reading unit, a linear noise appears in an image obtainedas a result of the reading. However, a factor causing the linear defectis not limited to the ink ejection failure in the nozzle. Accordingly,when the image on the sheet is read by the image reading unit, it isnecessary to reliably determine whether or not the noise included in theread image (electronic data) is a noise due to the ink ejection failurein the nozzle.

However, the technique described in Patent Literature 1 has a problem tobe solved such that, e.g., when dust is present at a position on thesheet different from an image formation surface of the sheet and thedust is read together with the image on the sheet by the image readingunit, whether or not a linear noise appearing in an image obtained as aresult of the reading is a noise due to the ink ejection failure in thenozzle cannot be determined.

An object of the present invention is to provide an image readingdevice, an image reading method, a non-transitory recording mediumstoring a computer readable program, and an image formation device whichallow whether or not a noise included in an image obtained as a resultof reading when an image on a sheet is to be read by an image readingunit is a noise due to an ink ejection failure in a nozzle to be morereliably determined.

To achieve at least one of the abovementioned objects, according to anaspect of the present invention, an image reading device includes: animage reading unit that reads an image formation surface of a sheet byusing a reducing optical system; and a control unit. The image readingunit includes a plurality of read sensors disposed at positionsdisplaced from each other in a main scanning direction. The plurality ofread sensors are disposed to be able to read the image formation surfaceby using at least a local region of the image formation surface as acommon region to be read. The control unit determines, based on a firstimage obtained by a first read sensor included in the plurality of readsensors and on a second image obtained by a second read sensor includedin the plurality of read sensors and different from the first readsensor, an amount of displacement of a noise resulting from a parallaxbetween the first read sensor and the second read sensor and determines,based on the amount of displacement, whether or not the noise is presenton the image formation surface.

To achieve at least one of the abovementioned objects, according to anaspect of the present invention, an image reading method using an imagereading device including an image reading unit that reads an imageformation surface of a sheet by using a reducing optical system andincludes a plurality of read sensors disposed at positions displacedfrom each other in a main scanning direction, the plurality of readsensors being disposed to be able to read the image formation surface byusing at least a local region of the image formation surface as a commonregion to be read, includes: determining, based on a first imageobtained by a first read sensor included in the plurality of readsensors and on a second image obtained by a second read sensor includedin the plurality of read sensors and different from the first readsensor, an amount of displacement of a noise resulting from a parallaxbetween the first read sensor and the second read sensor; anddetermining, based on the amount of displacement, whether or not thenoise is present on the image formation surface.

To achieve at least one of the abovementioned objects, according to anaspect of the present invention, a non-transitory recording mediumstoring a computer readable program for causing a computer of an imagereading device including an image reading unit that reads an imageformation surface of a sheet by using a reducing optical system andincludes a plurality of read sensors disposed at positions displacedfrom each other in a main scanning direction, the plurality of readsensors being disposed to be able to read the image formation surface byusing at least a local region of the image formation surface as a commonregion to be read, to execute: determining, based on a first imageobtained by a first read sensor included in the plurality of readsensors and on a second image obtained by a second read sensor includedin the plurality of read sensors and different from the first readsensor, an amount of displacement of a noise resulting from a parallaxbetween the first read sensor and the second read sensor; anddetermining, based on the amount of displacement, whether or not thenoise is present on the image formation surface.

To achieve at least one of the abovementioned objects, according to anaspect of the present invention, an image formation device that causeseach of a plurality of nozzles to eject ink to form an image on a sheet,includes: an image reading unit that reads an image formation surface ofthe sheet by using a reducing optical system; and a control unit. Theimage reading unit includes a plurality of read sensors disposed atpositions displaced from each other in a main scanning direction. Theplurality of read sensors are disposed to be able to read the imageformation surface by using at least a local region of the imageformation surface as a common region to be read. The control unitdetermines, based on a first image obtained by a first read sensorincluded in the plurality of read sensors and on a second image obtainedby a second read sensor included in the plurality of read sensors anddifferent from the first read sensor, an amount of displacement of anoise resulting from a parallax between the first read sensor and thesecond read sensor and determines, based on the amount of displacement,whether or not the noise is present on the image formation surface.

According to the present invention, when an image on a sheet is to beread by the image reading unit, it is possible to more reliablydetermine whether or not noise included in an image obtained as a resultof the reading is noise due to an ink ejection failure in a nozzle.

BRIEF DESCRIPTION OF THE DRAWINGS

The advantages and features provided by one or more embodiments of theinvention will become more fully understood from the detaileddescription given hereinbelow and the appended drawings which are givenby way of illustration only, and thus are not intended as a definitionof the limits of the present invention:

FIG. 1 is a schematic perspective view of an image formation deviceaccording to each of embodiments of the present invention;

FIG. 2 is a schematic side view illustrating an inner structure of theimage formation device illustrated in FIG. 1 ;

FIG. 3 is a schematic side view illustrating a portion of an imageformation unit illustrated in FIG. 2 in enlarged relation;

FIG. 4 is a diagram illustrating a state in which a plurality of ink-jetmodules are disposed;

FIG. 5 is a block diagram illustrating an example of a configuration ofa control system of the image formation device according to theembodiment of the present invention;

FIG. 6 is a schematic diagram illustrating a configuration of a mainportion of an image formation device according to a reference mode ofthe present invention;

FIG. 7 is a diagram illustrating a heat transfer prevention sheetcovering a conveyance drum;

FIG. 8 is a schematic diagram illustrating a configuration of a mainportion of the image formation device according to the embodiment of thepresent invention;

FIG. 9 is a schematic diagram for illustrating influence exerted by adistance from a read sensor to a noise generation source on a positionof a noise;

FIG. 10 is a flow chart illustrating an example of procedures ofprocessing by the image formation device according to the embodiment ofthe present invention;

FIG. 11 is a flow chart illustrating the procedure of noise extractionprocessing;

FIG. 12 is a flow chart illustrating the procedure of displacementdetection processing;

FIG. 13 is a schematic diagram illustrating image matching processing;

FIG. 14 is a schematic diagram illustrating a degree of matching betweenimages in the image matching processing;

FIG. 15 is a flow chart illustrating the procedure of distancecalculation processing;

FIG. 16 is a diagram illustrating a state in which an image of an objectserving as the generation source of the noise is focused on an imagingsurface of a first read sensor and on an imaging surface of a secondread sensor;

FIG. 17 is a diagram illustrating an example in which a distancecalculated by a distance calculation unit and the noise are stored inassociation with each other;

FIG. 18 is a diagram illustrating dimensional relations among individualportions of the image reading unit in the image formation deviceaccording to the embodiment of the present invention;

FIG. 19 is a flow chart illustrating the procedure of noisedetermination processing; and

FIG. 20 is a diagram illustrating an example of a threshold table.

DETAILED DESCRIPTION OF EMBODIMENTS

Hereinafter, one or more embodiments of the present invention will bedescribed with reference to the drawings. However, the scope of theinvention is not limited to the disclosed embodiments. Referring to thedrawings, a detailed description will be given of embodiments of thepresent invention. In the present specification and the drawings,components having substantially the same functions or configurations aredenoted by the same reference numerals, and a repeated descriptionthereof is omitted. In the following embodiments, paper will bedescribed as an example of a sheet. The paper include white paper, blackpaper, colored paper, and the like. The sheet is not limited to thepaper and may also be a resin sheet such as a transparent film, a fabricsheet, or the like. In the following description, the paper may also bereferred to as the sheet.

FIG. 1 is a schematic perspective view of an image formation deviceaccording to each of the embodiments of the present invention.

As illustrated in FIG. 1 , an image formation device 10 is an ink-jetrecording device which causes a plurality of nozzles to eject ink toform (record) an image on paper. The image formation device 10 may beeither a color image formation device or a monochrome image formationdevice. In the present embodiment, by way of example, a description willbe given of a case where the image formation device 10 is the colorimage formation device.

The image formation device 10 is a device which forms the image on thesheet by using a one-pass ink-jet method. In the one-pass ink-jetmethod, an image is formed without causing a recording head unit havinga plurality of nozzles to move in a main scanning direction. The mainscanning direction refers to a direction perpendicular to a sub-scanningdirection. The sub-scanning direction refers to a direction parallelwith a direction of conveyance of the paper. The one-pass ink-jet methodallows an image to be formed at a high speed in non-contact relation toa recording medium.

The image formation device 10 includes a paper feeding unit 11, an imageformation unit 12, a paper ejection unit 13, and an ink supply tank 14.The paper feeding unit 11 is a portion which supplies the paper as therecording medium. The image formation unit 12 is a portion which formsan image on the paper by using ink. The paper ejection unit 13 is aportion which ejects the paper after the image was formed thereon. Theink supply tank 14 is a tank for storing a predetermined amount of theink therein and supplying the ink to the image formation unit 12.

FIG. 2 is a schematic side view illustrating an inner structure of theimage formation device illustrated in FIG. 1 . The main scanningdirection mentioned above corresponds to a direction of depth in FIG. 2.

As illustrated in FIG. 2 , the paper feeding unit 11 has a paper feedingtray 11 a. On the paper feeding tray 11 a, sheets of paper 15 before animage is formed thereon or sheets of test paper having a predeterminedpattern or image formed thereon are stacked. The paper feeding unit 11sequentially separates the sheets of paper 15 stacked on the paperfeeding tray 11 a one by one from top to bottom and feeds the separatedpaper sheet.

The image formation unit 12 includes a conveyance drum 20 serving as aconveyance unit, a plurality of recording head units 21Y, 21M, 21C, and21K, a mist catcher 22, a UV irradiation unit 23, an image reading unit24, an inverting unit 25, and two larger and smaller conveyance rollers26 a and 26 b.

The conveyance drum 20 is rotatively provided. The conveyance drum 20rotates, while allowing the paper 15 fed from the paper feeding unit 11to be wound around an outer peripheral surface 20 a of the conveyancedrum 20. For example, the conveyance drum 20 causes the paper 15 to beattracted to the outer peripheral surface 20 a of the conveyance drum 20by suction of air, and rotates in this state to convey the paper 15 in aconveyance direction A. To implement such a conveyance method, in theouter peripheral surface 20 a of the conveyance drum 20, a plurality ofair suction holes (not shown) are formed.

The plurality of recording head units 21Y, 21M, 21C, and 21K form(record) an image on the paper 15 serving as the recording medium byusing inks in colors corresponding thereto. Specifically, the recordinghead unit 21Y uses yellow (Y) ink to form the image, while the recordinghead unit 21M uses magenta (M) ink to form the image. Meanwhile, therecording head unit 21C uses cyan (C) ink to form the image, while therecording head unit 21K uses black (K) ink to form the image. In thepresent embodiment, it is assumed by way of example that ultravioletcure inks are used.

Each of the recording head units 21Y, 21M, 21C, and 21K is disposed overthe conveyance drum 20 so as to face the outer peripheral surface 20 athereof. The individual recording head units 21Y, 21M, 21C, and 21K aredisposed at respective positions displaced from each other in acircumferential direction of the conveyance drum 20. In the presentembodiment, by way of example, the four recording head units 21Y, 21M,21C, and 21K are provided in the image formation unit 12.

The mist catcher 22 is disposed on a downstream side of the recordinghead unit 21K in the conveyance direction A in which the paper 15 isconveyed by the conveyance drum 20. The mist catcher 22 collects mistgenerated through the ejection of the inks from respective ink-jet headsprovided in the individual recording head units 21Y, 21M, 21C, and 21K.The UV irradiation unit 23 is disposed on the downstream side of themist catcher 22 in the conveyance direction A in which the paper 15 isconveyed. The UV irradiation unit 23 irradiates the paper 15 conveyedthereto by the rotation of the conveyance drum 20 with UV light to curethe inks on the paper 15. The inks on the paper 15 mentioned hereinrefer to the inks forming the image on the paper 15. The UV irradiationunit 23 functions as a fixing unit that fixes the image formed on thepaper 15 by using the inks. The fixation of the image by the fixing unitis not limited to that using the irradiation with the UV light, and mayalso be the fixation of the image using irradiation with an energy beamwhich may cure the inks depending on properties of the inks, thefixation of the image using heating of the paper 15 for drying the inks,or the like.

The image reading unit 24 is disposed on the downstream side of the UVirradiation unit 23 in the conveyance direction Ain which the paper 15is conveyed. The image reading unit 24 is a portion which performsreading on an image formation surface of the paper 15 by using areducing optical system. The image formation surface of the paper 15refers to a surface of the paper 15 facing each of the recording headunits 21Y, 21M 21C, and 21K when the paper 15 is conveyed, while beingattracted to the conveyance drum 20 by suction. On the image formationsurface of the paper 15, the image is formed by using each of therecording head units 21Y, 21M, 21C, and 21K. The inverting unit 25 is aportion which vertically inverts the paper 15 to form the image on eachside of the paper 15 or eject the paper 15 with the image formed on oneside of the paper 15 facing downward. The conveyance rollers 26 a and 26b refer to rollers which convey the paper 15 after the image was formedthereon toward the paper ejection unit 13.

The paper ejection unit 13 has a paper catch tray 13 a. Onto the papercatch tray 13 a, sheets of the paper 15 after the images were formedthereon are sequentially ejected.

FIG. 3 is a schematic side view illustrating a portion of the imageformation unit illustrated in FIG. 2 in enlarged relation. In thefollowing description, indexes Y, M, C, and K representing the colors ofthe inks are omitted except when the individual recording head units21Y, 21M, 21C, and 21K and the components thereof need to bedistinguished from each other by the colors of the inks.

As illustrated in FIG. 3 , a conveyance path (denoted by a broken linein the drawing) 31 for conveying the recording medium is formed on theouter peripheral surface 20 a of the conveyance drum 20. The paper 15(see FIG. 2 ) serving as the recording medium is conveyed with therotation of the conveyance drum 20, while being attracted to the outerperipheral surface 20 a of the conveyance drum 20 by suction. Therefore,the conveyance path 31 is formed along the outer peripheral surface 20 aof the conveyance drum 20.

Halfway in the conveyance path 31, image recording positions P1, P2, P3,P4, and P5 corresponding to the individual recording head units 21Y,21M, 21C, and 21K are present. The image recording position P1 refers toa position at which the recording head unit 21Y records (forms) theimage, while the image recording position P2 refers to a position atwhich the recording head unit 21M records the image. The image recordingposition P3 refers to a position at which the recording head unit 21Crecords the image, while the image recording position P4 refers to aposition at which the recording head unit 21K records the image.

When the paper 15 is conveyed along the conveyance path 31, a leadingend of the paper 15 first passes through the image recording position P1corresponding to the recording head unit 21Y, and then sequentiallypasses through the image recording positions P2, P3, and P4corresponding to the other recording head units 21M, 21C, and 21K. Then,when the four recording head units 21Y, 21M, 21C, and 21K eject the inkswith predetermined timings, the yellow ink adheres to the paper 15 atthe image recording position P1, and the magenta ink adheres to thepaper 15 at the image recording position P2. Then, the cyan ink adheresto the paper 15 at the image recording position P3, and the black inkadheres to the paper 15 at the image recording position P4.

The recording head unit 21Y includes an ink-jet head 32Y and a carriage33Y holding the ink-jet head 32Y. The ink-jet head 32Y has an inkejection surface 34Y and ejects the ink from nozzles 244 (see FIG. 4 )provided in the ink ejection surface 34Y. The ink ejection surface 34Yof the ink-jet head 32Y is disposed, while facing the outer peripheralsurface 20 a of the conveyance drum 20.

In the ink ejection surface 34Y of the ink-jet head 32Y, as illustratedin, e.g., FIG. 4 , a plurality of ink-jet modules 243 are provided. Theplurality of ink-jet modules 243 are arranged in a staggeredconfiguration. In each of the ink-jet modules 243, two nozzle units 242are provided. In each of the nozzle units 242, the plurality of nozzles244 are provided. The nozzles 244 are portions which eject the ink. Whenthe image is formed on the paper 15 by the ink-jet head 32Y, the nozzles244 are selected based on image data specified by a print job, and theink is ejected from the selected nozzles 244 toward the paper 15. Notethat an X-direction illustrated in FIG. 4 corresponds to the mainscanning direction, while a Y-direction illustrated in FIG. 4corresponds to the sub-scanning direction.

Similarly to the recording head unit 21Y described above, the recordinghead unit 21M includes an ink-jet head 32M and a carriage 33M holdingthe ink-jet head 32M. Also, the recording head unit 21C includes anink-jet head 32C and a carriage 33C holding the ink-jet head 32C, whilethe recording head unit 21K includes an ink-jet head 32K and a carriage33K holding the ink-jet head 32K.

FIG. 5 is a block diagram illustrating an example of a configuration ofa control system of the image formation device according to theembodiment of the present invention.

As illustrated in FIG. 5 , the image formation device 10 includes notonly the paper feeding unit 11, the image formation unit 12, and thepaper ejection unit 13, but also a control unit 51, an operation/displayunit 53, a storage unit 58, and a communication unit 59.

The image formation unit 12 includes, in addition to the image readingunit 24 described above, a head drive unit 55 and a conveyance motor 57.The image reading unit 24 includes a plurality of read sensors 62. Eachof the read sensors 62 is an image sensor that optically reads an imagevia the reducing optical system described later, which is specifically alinear image sensor in which photoelectric conversion elements arearranged in a linear configuration. The image reading unit 24 has asurface serving as a reference when the image formation surface of thepaper 15 is read, i.e., a read reference surface. The read referencesurface of the image reading unit 24 is set at a position at a givendistance apart from an imaging surface of each of the read sensors 62.

Each of the read sensors 62 is formed of, e.g., a CCD (Charge CoupledDevice) sensor, a CMOS (Complementary Metal Oxide Semiconductor) sensor,or the like. When reading the image formed on the paper 15, the readsensor 62 outputs image data obtained as a result of the reading atthree wavelengths of, e.g., R (red), G (green), and B (blue) light. Interms of obtaining the effects of the present embodiment, the readsensor 62 may be either a color image sensor or a monochrome imagesensor.

The head drive unit 55 independently drives each of the ink-jet heads32Y, 32M, 32C, and 32K based on a control instruction given thereto fromthe control unit 51. The conveyance motor 57 includes a motor forconveying the paper 15 and rotating the conveyance drum 20. The drivingof the conveyance motor 57 is controlled by the control unit 51.

The control unit 51 includes a CPU (Central Processing Unit) 61, a ROM(Read Only Memory) 63, and a RAM (Random Access Memory) 65. The controlunit 51 causes the CPU 61 to read a predetermined control program fromthe ROM 63 into the RAM 65 and execute the predetermined control programto perform overall control of operations of the individual units of theimage formation device 10. The control unit 51 transmits/receivesvarious data to/from an external device (such as, e.g., a personalcomputer) connected to a communication network not shown. When receivingthe print job from the operation/display unit 53 or the external device,the control unit 51 controls the operations of the individual units ofthe image formation device 10 so as to form an image on the paper 15based on the image data specified by the print job.

The control unit 51 also determines an amount of displacement of a noiseresulting from a parallax between the first read sensor 62 a and thesecond read sensor 62 b based on a first image 81 a obtained by a firstread sensor 62 a described later and on a second image 81 b obtained bya second read sensor 62 b described above to determine whether or notthe noise is present on the image formation surface of the paper 15based on the amount of displacement. As functional units therefor, thecontrol unit 51 includes a noise extraction unit 71, a displacementdetection unit 73, a distance calculation unit 74, and a noisedetermination unit 75. The control unit 51 further includes an imagecorrection unit 76 and an image quality adjustment unit 77. Each of thefunctional units (71, 73, 74, 75, 76, and 77) is implemented by the CPU61 by reading a predetermined program from the ROM 63 into the RAM 65and executing the predetermined program.

The noise extraction unit 71 is a portion which extracts a noise from animage (hereinafter referred to also as a “first image”) obtained by apredetermined one of the plurality of read sensors 62 described above.The displacement detection unit 73 is a portion which specifies aposition on an image (hereinafter referred to also as a “second image”)obtained by another read sensor 62 different from the predetermined readsensor 62 described above at which the noise extracted by the noiseextraction unit 71 is present and also detects, based on informationabout the specified position, an amount of displacement corresponding toan amount of displacement (parallax) between a position (in the mainscanning direction) on the first image at which the noise is present andthe position (in the main scanning direction) on the second image atwhich the noise is present.

The distance calculation unit 74 is a portion which calculates, based onthe above-mentioned amount of displacement detected by the displacementdetection unit 73, a distance from the read reference surface of theimage reading unit 24 to a generation source of the noise. The noisedetermination unit 75 is a portion which determines, based on theabove-mentioned distance calculated by the distance calculation unit 74,whether or not the noise is present on the image formation surface.

The image correction unit 76 is a portion which corrects a missingportion of an image due to a missing nozzle defect. The image qualityadjustment unit 77 is a portion which adjusts a quality of the imageformed on the paper 15. The image quality adjustment unit 77 adjusts,e.g., a density, a position, an inclination, and the like of the imageas adjustment parameters associated with the quality of the image, i.e.,image quality. Details of each of the functional units will be describedlater.

The operation/display unit 53 is formed of, e.g., a liquid crystaldisplay with a touch panel. The operation/display unit 53 has a functionof displaying various information to a user and a function of receivingvarious input operations from the user. The information displayed on theoperation/display unit 53 includes various operation screens, a settingscreen, a reporting screen, and the like. The input operations performedby the user by using the operation/display unit 53 include an operationof giving an instruction to execute the print job, an operation ofselecting (specifying) the image data for which the print job isintended, an operation of selecting (specifying) a type of the paper 15to be used for the print job, an operation of selecting conditions to beapplied to the print job, and the like.

The storage unit 58 is formed of, e.g., a nonvolatile semiconductormemory (so-called flash memory), a hard disk, or the like. The storageunit 58 stores therein control data to be referred to by the controlunit 51 when performing various control processing and other data.

The communication unit 59 is communicatively connected to the externaldevice (such as, e.g., a personal computer) via the communicationnetwork not shown to transmit/receive various data to/from the externaldevice. The communication unit 59 is formed of various interfaces suchas, e.g., NIC (Network Interface Card), a MODEM (MOdulator-DEModulator),and a USB (Universal Serial Bus).

A description will be given herein of a reference mode of the presentinvention.

FIG. 6 is a schematic diagram illustrating a configuration of a mainportion of an image formation device according to the reference mode ofthe present invention.

As illustrated in FIG. 6 , the outer peripheral surface 20 a (see FIG. 2) of the conveyance drum 20 is covered with a heat transfer preventionsheet 27. The heat transfer prevention sheet 27 is interposed betweenthe paper 15 and the conveyance drum 20 to prevent heat transfer betweenthe paper 15 and the conveyance drum 20 when the paper 15 is conveyedwhile being attracted to the outer peripheral surface 20 a of theconveyance drum 20 by suction. Since a viscosity of the ink ejected froman ink-jet head 32 is susceptible to a temperature, to maintain anexcellent quality of the image formed on the paper 15, it is required tocontrol respective temperatures of the ink-jet head 32, the paper 15,and the conveyance drum 20 to appropriate levels. At that time, whenheat easily transfers between the conveyance drum 20 and the paper 15and between the conveyance drum 20 and the ink-jet head 32, it isdifficult to appropriately control the temperatures of the individualunits. Accordingly, by covering the outer peripheral surface 20 a of theconveyance drum 20 with the heat transfer prevention sheet 27, heat isprevented from transferring between the conveyance drum 20 and the paper15 and between the conveyance drum 20 and the ink-jet head 32.

The heat transfer prevention sheet 27 is formed of a sheet having a lowheat conductivity. Specifically, the heat transfer prevention sheet 27is formed of a resin sheet made of a synthetic resin or the like and,more specifically, the heat transfer prevention sheet 27 is formed of afluorine resin sheet or the like. The conveyance drum 20 attracts thepaper 15 by suction by using a sucking force of air. Accordingly, theheat transfer prevention sheet 27 is formed of a porous sheet, a porousfilm, or the like formed in a lattice pattern (netlike pattern) asillustrated in, e.g., FIG. 7 so as to have an appropriate airpermeability.

Note that, in FIG. 7 , the outer peripheral surface 20 a of theconveyance drum 20 is covered with the heat transfer prevention sheet27, and air suction holes 20 b of the conveyance drum 20 aretransparently seen in a spotted pattern through the heat transferprevention sheet 27. Also, in FIG. 7 , a portion of the heat transferprevention sheet 27 is displayed in enlarged relation on the right sideof the drawing.

An image reading unit 240 is a portion which optically reads an imageformation surface 15 c of the paper 15. FIG. 6 illustrates a state inwhich an image 16 is formed on the image formation surface 15 c of thepaper 15, and a missing nozzle defect 17 is present in the image 16. Themissing nozzle defect 17 is a missing portion of the image 16 resultingfrom an ink ejection failure in any of the nozzles 244.

The image reading unit 240 includes the read sensor 62, a reducingoptical system 64, a reading plate 66, and a light source (not shown).In the reference mode, the only one read sensor 62 is provided in theimage reading unit 240, and the only one reducing optical system 64 isprovided correspondingly thereto.

The reducing optical system 64 includes a condensing lens not shown. Thereducing optical system 64 condenses light reflected from a surface tobe read with the condensing lens to form an image on the imaging surfaceof the read sensor 62. The reference mode assumes a case in which thesurface to be read is the image formation surface 15 c of the paper 15.The reading plate 66 is formed of, e.g., a transparent glass plate. Thereference mode assumes a case where a foreign substance 80 adheres tothe reading plate 66. Examples of the foreign substance 80 include mist,dust, paper powder, and the like. The light source irradiates thesurface to be read with reading light (e.g., white light), and is formedof a linear lighting device or the like. The linear lighting deviceemits linear light, and is formed by using a light emitting diode or thelike.

In the image formation device according to the reference mode of thepresent invention, when the read sensor 62 reads the image 16 on thepaper 15 conveyed by the conveyance drum 20 via the reducing opticalsystem 64, the missing nozzle defect 17 present in the image 16 and theforeign substance 80 adhering to the reading plate 66 are read togetherwith the image 16. In addition, when the paper 15 is thin paper or thelike, the read sensor 62 reads the lattice pattern of the heat transferprevention sheet 27 through the thin paper. When the read sensor 62actually reads the image formation surface 15 c of the paper 15, animage 81 as illustrated on a right side of FIG. 6 is obtained. The image81 is electronic data of an imaged image obtained as a result of thereading by the read sensor 62, and can also be referred to as the imagedata.

The image 81 obtained by the read sensor 62 includes a noise 82 due tothe foreign substance 80, a noise 83 due to the missing nozzle defect17, and a noise 84 due to the lattice pattern of the heat transferprevention sheet 27. These noises 82, 83, and 84 appear while beingsuperimposed on the one image 81. The noise 82 due to the foreignsubstance 80 is a linear noise extending in parallel with thesub-scanning direction Y, and the noise 83 due to the missing nozzledefect 17 is also a linear noise extending in parallel with thesub-scanning direction Y. The noise 84 due to the lattice pattern of theheat transfer prevention sheet 27 is noise appearing in the form ofmoire.

As a technique of preventing degradation of an image quality due to themissing nozzle defect, a technique of specifying a position of themissing nozzle defect to estimate a position of a faulty nozzle andcompensate for an image to be formed by the faulty nozzle by using anozzle adjacent to the faulty nozzle or a nozzle serving as a substitutefor the faulty nozzle is known. The known technique can eliminate orreduce the missing nozzle defect. However, to use this known technique,it is required to reliably specify the position of the missing nozzledefect.

However, in the image formation device according to the reference modeof the present invention, when the image 81 obtained by the read sensor62 includes the noises 82 and 84 other than the noise 83 due to themissing nozzle defect 17, it is difficult to reliably specify theposition of the missing nozzle defect 17. Specifically, each of thenoise 83 due to the missing nozzle defect 17 and the noise 82 due to theforeign substance 80 is present as linear noise in the image 81.Consequently, it is impossible to determine whether or not each of thenoise 82 and the noise 83 is the noise due to the missing noise defect17. In addition, when the paper 15 is thin paper or the like, the noise84 due to the heat transfer prevention sheet 27 is superimposed on theimage 81, and therefore it is increasingly difficult to determine thenoise.

FIG. 8 is a schematic diagram illustrating a configuration of a mainportion of the image formation device according to the embodiment of thepresent invention. As illustrated in FIG. 8 , the image reading unit 24includes the two read sensors 62 a and 62 b, two reducing opticalsystems 64 a and 64 b, the reading plate 66, and the light source (notshown). The reading plate 66 and the light source are as described abovein the reference mode. By way of example, the embodiment of the presentinvention will describe a case where the image reading unit 24 includesthe two read sensors 62 a and 62 b, but the number of the read sensors62 included in the image reading unit 24 may also be 3 or more.

The read sensors 62 a and 62 b are disposed at positions displaced fromeach other in the main scanning direction X. A read reference surface 24a of the image reading unit 24 is set between imaging surfaces 90 a and90 b (see FIG. 16 ) of the read sensors 62 a and 62 b and the readingplate 66. The image reading unit 24 reads an image in accordance with aso-called stereo camera method which simultaneously photographs anobject to be photographed in different directions by using the two readsensors 62 a and 62 b. In the description given below, the read sensors62 a and 62 b are referred to as the “first read sensor 62 a” and the“second sensor 62 b”, respectively. When there is no need toparticularly distinguish the first and second read sensors 62 a and 62 bfrom each other, each of the read sensors 62 a and 62 b is referred toas the “read sensor 62”. Note that, in the sub-scanning direction, theread sensors 62 a and 62 b are preferably at the same position, but mayalso be at different positions. Also, in a vertical direction, the readsensors 62 a and 62 b are preferably at the same position, but may alsobe at different positions. The vertical direction mentioned hereinrefers to a direction perpendicular to each of the main scanningdirection X and the sub-scanning direction.

The reducing optical system 64 a is provided correspondingly to thefirst read sensor 62 a, while the reducing optical system 64 b isprovided correspondingly to the second read sensor 62 b. The reducingoptical system 64 a is an optical system which condenses light reflectedfrom the surface to be read with a condensing lens to form an image onan imaging surface of the first read sensor 62 a. The reducing opticalsystem 64 b is an optical system which condenses the light reflectedfrom the surface to be read with a condensing lens to form an image onan imaging surface of the second read sensor 62 b. In the descriptiongiven below, the reducing optical system 64 a is referred to as the“first reducing optical system 64 a”, while the reducing optical system64 b is referred to as the “second reducing optical system 64 b”.

The first read sensor 62 a reads the surface to be read via the firstreducing optical system 64 a to generate the first image 81 acorresponding to the image data showing a result of the reading. Thesecond read sensor 62 b reads the surface to be read via the secondreducing optical system 64 b to generate the second image 81 bcorresponding to the image data showing a result of the reading. Thesurface to be read is as described above in the reference mode.

A viewing angle in the main scanning direction X resulting from thefirst read sensor 62 a is set such that, when the paper 15 having amaximum width that can be conveyed by the conveyance drum 20 isconveyed, an end portion 15 a of the paper 15 in a width directionthereof is located at an end of a viewing field. Meanwhile, a viewingangle in the main scanning direction X resulting from the second readsensor 62 b is set such that another end portion 15 b of theabove-mentioned paper 15 having the maximum width in the width directionthereof is located at an end of a viewing field. The viewing field ofthe first read sensor 62 a in the main scanning direction X is set widerthan a width dimension of the above-mentioned paper 15 having themaximum width, and the viewing field of the second read sensor 62 b inthe main scanning direction X is also set wider than the width dimensionof the above-mentioned paper 15 having the maximum width.

Accordingly, in the image reading unit 24, the first read sensor 62 a isconfigured to be able to read an entire region of the image 16 formed onthe paper 15, and the second read sensor 62 b is also configured to beable to read the entire region of the image 16 formed on the paper 15.In other words, the first read sensor 62 a and the second read sensor 62b are disposed to be able to read an entire region of the imageformation surface 15 c of the paper 15 as a common region to be read.

Note that, by way of example, the present embodiment will describe acase where the entire region of the image formation surface 15 c of thepaper 15 is used as the common region to be read. However, the commonregion to be read when the first read sensor 62 a and the second readsensor 62 b read the image formation surface 15 c of the paper 15 mayalso be a portion of the image formation surface 15 c.

Subsequently, referring to FIGS. 8 and 9 , a description will be givenof influence exerted by distances from the read sensors 62 a and 62 b tothe generation source of the noise (hereinafter referred to also as the“noise generation source”) in a direction perpendicular to the mainscanning direction X on a position of the noise appearing in the firstimage 81 a and on a position of the noise appearing in the second image81 b.

First, as illustrated in FIG. 8 , the image reading unit 24 reads animage in accordance with the stereo camera method using the plurality of(two in the present mode example) read sensors 62 a and 62 b.Accordingly, with respect to a given noise generation source, a positionon the imaging surface of the first read sensor 62 a at which an imageof the noise generation source is formed and a position on the imagingsurface of the second read sensor 62 b at which an image of the noisegeneration source is formed are displaced from each other in the mainscanning direction X. Consequently, with regard to the given noisegeneration source, a position of noise observed in the first image 81 aand a position of noise observed in the second image 81 b are alsodisplaced from each other in the main scanning direction X.

As illustrated in FIGS. 8 and 9 , when the foreign substance 80 adheringto the reading plate 66 causes a noise 82 a in the first image 81 a andcauses a noise 82 b in the second image 81 b, an amount of displacementbetween the noises 82 a and 82 b in the main scanning direction X isassumedly denoted by D1 (mm). Meanwhile, when the missing nozzle defect17 caused on the paper 15 by an ink ejection failure in one of thenozzles 244 causes a noise 83 a in the first image 81 a and causes anoise 83 b in the second image 81 b, an amount of displacement betweenthe noises 83 a and 83 b in the main scanning direction X is assumedlydenoted by D2 (mm). In addition, when the surface pattern of the heattransfer prevention sheet 27 seen through the paper 15 causes a noise 84a in the first image 81 a and causes a noise 84 b in the second image 81b, a displacement between the noises 84 a and 84 b in the main scanningdirection X is assumedly denoted by D3 (mm). In that case, a magnituderelationship among the amounts of displacement D1, D2, and D3 is givenby D1>D2>D3. In other words, with regard to a given noise generationsource, an amount of displacement between a position of a noise observedin the first image 81 a and a position of a noise observed in the secondimage 81 b is larger as the noise generation source viewed from the readsensors 62 a and 62 b is closer.

FIG. 10 is a flow chart illustrating an example of procedures ofprocessing by the image formation device according to the embodiment ofthe present invention.

As illustrated in FIG. 10 , the image formation device 10 sequentiallyperforms image reading processing S1, noise extraction processing S2,displacement detection processing S3, distance calculation processingS4, and noise determination processing S5. A description will be givenbelow of each processing.

(Image Reading Processing)

The image reading processing S1 is processing to be performed by theimage reading unit 24. The image reading unit 24 reads the imageformation surface 15 c of the paper 15 conveyed by the conveyance drum20 with the first read sensor 62 a and the second read sensor 62 b. Atthis time, the first image 81 a obtained by the first read sensor 62 ais stored in the RAM 65, and the second image 81 b obtained by thesecond read sensor 62 b is also stored in the RAM 65.

(Noise Extraction Processing)

The noise extraction processing S2 is processing to be performed by thenoise extraction unit 71.

The noise extraction unit 71 extracts at least any one of a noise due tomist, dust, or paper powder present between the image formation surface15 c of the paper 15 and the read reference surface 24 a of the imagereading unit 24, a noise due to the missing noise defect 17 appearing onthe image formation surface 15 c of the paper 15, a noise due to theconveyance drum 20 serving as the sheet conveyer, and a noise due to theheat transfer prevention sheet 27 covering the surface of the conveyancedrum 20.

FIG. 11 is a flow chart illustrating the procedure of the noiseextraction processing S2.

First, the noise extraction unit 71 performs edge detection processingby using the first image 81 a stored together with the second image 81 bin the RAM 65 as described above (Step S21). The first image 81 a to besubjected to edge detection is an image having three channels of R(red), G (green), and B (blue), and according the noise extraction unit71 first converts the first image 81 a into one channel through grayscale conversion. As a result, the first image 81 a represented by blackand white densities is obtained. At this time, the second image 81 b isalso converted into one channel through the gray scale conversion. Then,the noise extraction unit 71 performs primary differential filterprocessing and secondary differential filter processing on the firstimage 81 a obtained through the gray scale conversion to detect edgespresent in the first image 81 a.

Then, the noise extraction unit 71 determines, based on a result of theedge detection processing, whether or not the first image 81 a includesa linear noise (Step S22). The “linear noise” mentioned herein means alinear noise extending in parallel with the sub-scanning direction Y.Accordingly, when at least one of the edges detected by the edgedetection processing is an edge continuously extending in thesub-scanning direction Y, i.e., linear noise, the noise extraction unit71 determines that there is a linear noise (YES is given as an answer inStep S22), subsequently, processing in Step S23 is performed, and thenthe noise extraction processing S2 is ended. When there is no edgecontinuously extending in the sub-scanning direction Y, the noiseextraction unit 71 determines that there is no linear noise (NO is givenas an answer in Step S22), and the noise extraction processing S2 isended.

In Step S23, the noise extraction unit 71 determines a region includingthe linear noise described above to be an image matching region. Theimage matching region is a local region of the first image 81 a, and asize of the image matching region corresponds to a size of an imageregion to be shifted over the second image 81 b in image matchingprocessing described later. The size of the image matching region isdefined by a width of the region in the main scanning direction X and aheight of the region in the sub-scanning direction Y, and the width andheight of the region are determined in Step S23. In Step S23, when thesize of the image matching region is determined by a width of the regioncorresponding to M pixels in the main scanning direction X and by aheight of the region corresponding to N pixels in the sub-scanningdirection Y, in the image matching processing described later, an imageincluding the linear noise and having an M pixel×N pixel size over thefirst image 81 a is shifted over the second image 81 b. The imagematching processing will be described later in detail.

When a plurality of the linear noises are extracted from the first image81 a by the noise extraction processing S2 described above, the noiseextraction unit 71 may determine the image matching region for each ofthe linear noises but, more preferably, a region including the pluralityof linear noises may appropriately be determined to be the imagematching region, the reason of which will be described later.

(Displacement Detection Processing)

The displacement detection processing S3 is processing to be performedby the displacement detection unit 73.

FIG. 12 is a flow chart illustrating the procedure of the displacementdetection processing S3.

First, the displacement detection unit 73 reads, from the storage unit58, an overlap width Wo (see FIG. 13 ) over which the first image 81 aand the second image 81 b overlap each other in the main scanningdirection X (Step S31). As illustrated in FIG. 8 described above, theoverlap width Wo corresponds to a width over which respective regions tobe read by the first read sensor 62 a and the second read sensor 62 boverlap each other in the main scanning direction X when the imageformation surface 15 c of the paper 15 having the maximum width is readby the first and second read sensors 62 a and 62 b, i.e., the entirewidth of the paper 15. A specific numerical value of the overlap widthWo is set in advance depending on the maximum width of the paper 15 usedin the image formation device 10 and on a positional relationshipbetween the conveyance drum 20 and the image reading unit 24, i.e.,machine design values of the image formation unit 12. Note that, in thefirst image 81 a and the second image 81 b each illustrated in FIG. 13 ,an image portion falling out of the overlap width Wo is the imageportion obtainable through reading of a portion falling out of the paper15 having the maximum width in the main scanning direction X by thefirst read sensor 62 a and the second read sensor 62 b.

Next, the displacement detection unit 73 reads, from the storage unit58, the number of overlap lines Ho (see FIG. 13 ) in which the firstimage 81 a and the second image 81 b overlap each other in thesub-scanning direction Y (Step S32). The number of the overlap lines Hocorresponds to the number of lines in which the respective regions to beread by the first read sensor 62 a and the second read sensor 62 boverlap each other in the sub-scanning direction Y, i.e., a height ofthe image when the image formation surface 15 c of the paper 15 conveyedby the conveyance drum 20 is read by the first read sensor 62 a and thesecond read sensor 62 b. When the maximum length of the paper 15 used inthe image formation device 10 in the sub-scanning direction Y is assumedto be Lmax, the number of the overlap lines Ho is set in advance underthe condition that the number of the overlap lines Ho is less than thenumber of lines required by the read sensor 62 (62 a or 62 b) to readthe entire region of the image formation surface 15 c of the paper 15having the maximum length Lmax. The number of the overlap lines Ho isalso set in advance under the condition that the number of the overlaplines Ho is equal to or larger than the number of lines required todetermine the amount of displacement of the noise resulting from theparallax between the first read sensor 62 a and the second read sensor62 b.

As illustrated in FIG. 13 , the first image 81 a and the second image 81b have respective regions (Wo×Ho) each defined by the overlap width Woand the number of overlap lines Ho which are at positions opposite toeach other in the main scanning direction X. In other words, in thefirst image 81 a, the position of the region (Wo×Ho) is located on oneside (right side in FIG. 13 ) in the main scanning direction X while, inthe second image 81 b, the position of the region (Wo×Ho) is located onthe other side (left side in FIG. 13 ) in the main scanning direction X.This is because, as illustrated in FIG. 8 , the first read sensor 62 aand the second read sensor 62 b are disposed at positions displaced fromeach other in the main scanning direction X and, due to the displacementbetween the sensors, positions of images representing a result of thereading of the image formation surface 15 c of the paper 15 are oppositeto each other in the main scanning direction X.

Then, the displacement detection unit 73 performs, for the first image81 a and the second image 81 b, image matching processing on the regions(Wo×Ho) defined by the overlap width Wo and the number of overlap linesHo which are read from the storage unit 58 in Steps S31 and S32described above (Steps S33 to S35). As an example of the image matchingprocessing, region-based matching can be used.

In the image matching processing, first, the displacement detection unit73 uses, as a template image, an M pixel×N pixel image 85 including thelinear noises extracted by the noise extraction processing S2 describedabove, and shifts the image 85 over the second image 81 b, asillustrated in FIG. 13 (Step S33). Values of the individual pixels inthe image 85 represent density values (pixel values) after the grayscale conversion. The same applies also to an image 86 described later.A size of a region of the second image 81 b over which the image 85 isshifted is given by Overlap Width Wo×Number of Overlap Lines Ho. Aminimum amount of shifting per shifting operation corresponds to onepixel in each of the main scanning direction X and the sub-scanningdirection Y. The amount of shifting per shifting operation may be setappropriately in consideration of accuracy of the image matchingprocessing and a time required for the image matching processing. In thepresent embodiment, by way of example, it is assumed that the amount ofshifting per shifting operation is one pixel (Ws) in the main scanningdirection X and, after the image 85 is shifted over the second image 81b from a left end of the region (Wo×Ho) to a right end thereof, theimage 85 is shifted by one pixel in the sub-scanning direction Y.

Then, the displacement detection unit 73 calculates a degree of matchingbetween the image 85 serving as the template image and the image (targetimage) 86 to be subjected to matching with the image 85 at apost-shifting position on the second image 81 b (Step S34). Thepost-shifting position corresponds to a position to which the image 85is shifted over the second image 81 b. As a method of calculating thedegree of matching, for example, any of three methods described belowmay be used appropriately. The first method is a calculation methodreferred to as SSD (Sum of Squared Differences) which uses a square sumof pixel value differences as an index. The second method is acalculation method referred to as a SAD (Sum of Absolute Differences)which uses absolute value differences of pixel value differences as anindex. The third method is a calculation method referred to as NCC(Normalized Cross Correlation) which uses a normalized function. In thepresent embodiment, it is assumed that, by way of example, the degree ofmatching between the images is calculated by a SAD-based calculationmethod.

In Step S34, the displacement detection unit 73 also stores, in thestorage unit 58 (or the RAM 65), the degree of matching calculated ateach of the post-shifting positions in association with the amount ofshifting at which the degree of matching is obtained. The amount ofshifting is stored in the storage unit 58 as information representingrespective approximate distances over which the image 85 is shifted inthe main scanning direction X and the sub-scanning direction Y from areference position when, e.g., an upper left pixel position in theregion (Xo×Ho) of the second image 81 b is assumed to be the referenceposition.

Then, the displacement detection unit 73 determines whether or not theshifting of the image 85 is completed in the region (Wo×Ho) of thesecond image 81 b (Step S35). Then, when the shifting is not completed,the processing returns to Step S33 described above, and, when theshifting is completed, the processing advances to subsequent Step S36.

In Step S36, the displacement detection unit 73 extracts the amount ofshifting stored in the storage unit 58 in association with the highestone of the degrees of matching. In the SAD-based calculation method, thedegree of matching between the image 85 serving as the template imageand the image 86 serving as the target image is higher as aSAD-calculated value is smaller. Accordingly, in Step S36, thedisplacement detection unit 73 extracts the amount of shifting stored inthe storage unit 58 in association with a minimum SAD-calculated valueamong the degrees of matching stored in the storage unit 58.

Then, the displacement detection unit 73 specifies, based on the amountof shifting extracted in Step S36 described above, a position on thesecond image 81 b at which the noise (linear noise) extracted by thenoise extraction processing S2 described above is present (Step S37).

Then, the displacement detection unit 73 detects an amount ofdisplacement between a position on the first image 81 a at which thenoise extracted by the noise extraction processing S2 is present and theposition on the second image 81 b at which the noise extracted by thenoise extraction processing S2 is present, i.e., the amount ofdisplacement of the noise resulting from the parallax between the firstread sensor 62 a and the second read sensor 62 b (Step S38). The amountof displacement of the noise resulting from the parallax is defined by alength direction of the imaging surface of the second read sensor 62 b.The length direction of the imaging surface means a direction in whichthe photoelectric conversion elements are arranged.

In a case of performing the matching processing on the first image 81 aand the second image 81 b by the displacement detection processing S3described above as illustrated in FIG. 14 , an error (degree ofmatching) when a position of a linear noise 87 a present on the firstimage 81 a does not match a position of a linear noise 87 b present onthe second image 81 b in the main scanning direction X and an error(degree of matching) when the position of the linear noise 87 a matchesthe position of the linear noise 87 b in the main scanning direction Xvary depending on the amount of shifting of the image. Consequently, inStep S36, the displacement detection unit 73 extracts the amount ofshifting at which the error when the position of the linear noise 87 apresent on the first image 81 a matches the position of the linear noise87 b present on the second image 81 b, i.e., the degree of matching hasa smallest value (minimum value).

When a plurality of linear noises are extracted from the first image 81a by the noise extraction processing S2 previously described and aregion including the plurality of linear noises is determined to be theimage matching region, the displacement detection unit 73 mayappropriately specify a position on the second image 81 b at which theimage has a highest degree of matching with the image in the imagematching region as a position at which the plurality of linear noisesdescribed above are present. The position at which the image has thehighest degree of matching mentioned herein refers to a position on theimage at which the SAD-calculated value is minimum, i.e., a position onthe image corresponding to the amount of shifting at which the degree ofmatching between the image 85 serving as the template image and theimage 86 serving as the target image is highest.

By thus collectively specifying the positions of the plurality of linearnoises on the second image 81 b, it is less likely to confuse a positionof a given noise with a position of another noise present in thevicinity thereof than when the position of each of the plurality oflinear noises is individually specified, and accordingly it is possibleto more reliably specify the positions of the noises.

(Distance Calculation Processing)

The distance calculation processing S4 is processing to be performed bythe distance calculation unit 74.

FIG. 15 is a flow chart illustrating the procedure of the distancecalculation processing S4.

First, the distance calculation unit 74 calculates, based on acalculation formula using the amount of displacement of the noisedetected by the displacement detection processing S3 described above asa parameter, the distance from the read reference surface 24 a of theimage reading unit 24 to the generation source of the noise (Step S41).Referring to FIG. 16 , a detailed description will be given below of thecalculation processing.

FIG. 16 illustrates a state in which an image of an object 89corresponding to the generation source of the noise is focused on theimaging surface 90 a of the first read sensor 62 a and on the imagingsurface 90 b of the second read sensor 62 b.

In FIG. 16 , a reference mark B denotes a distance (hereinafter referredto also as a “camera-to-camera distance”) between the first read sensor62 a and the second read sensor 62 b in the main scanning direction X.The unit of the distance B is assumed to be millimeter.

A reference mark F denotes a focal point distance between the firstreducing optical system 64 a and the second reducing optical system 64b. The focal point distance F is a distance determined by opticalcharacteristics of the condensing lenses used in the first reducingoptical system 64 a and the second reducing optical system 64 b, whichis known information. Specifically, the focal point distance Fcorresponds to each of distances from respective centers (main points)of the condensing lenses of the individual reducing optical systems 64 aand 64 b to the imaging surfaces 90 a and 90 b. Meanwhile, the readreference surface 24 a of the image reading unit 24 is set at a positionspaced apart from each of the imaging surfaces 90 a and 90 b by thefocal point distance F in a direction perpendicular to the main scanningdirection X. The focal point distance F may appropriately be stored inadvance in the storage unit 58 and read as required from the storageunit 58. The unit of the focal distance F is assumed to be millimeter.

A reference mark D denotes an amount of displacement between a positionon the imaging surface 90 a of the first read sensor 62 a at which animage of the object 89 is formed and a position on the imaging surface90 b of the second read sensor 62 b at which an image of the object 89is formed, i.e., an amount of displacement between the images of theobject 89 resulting from the parallax between the first read sensor 62 aand the second read sensor 62 b. The unit of the amount of displacementD resulting from the parallax is assumed to be millimeter.

A reference mark Z denotes a distance from the read reference surface 24a of the image reading unit 24 to the object 89 in a directionperpendicular to the main scanning direction X. The unit of the distanceZ is assumed to be millimeter. The distance Z can be determined based onExpression (1) below because a triangle 91 having three points P11, P12,and P13 as vertices thereof and a triangle 92 having three points P13,P14, and P15 having vertices thereof have a synonymous relationshiptherebetween.Z=B×F/D  (1)

The distance Z calculated based on Expression (1) above corresponds tothe distance from the read reference surface 24 a of the image readingunit 24 to the noise generation source when the object 89 is the noisegeneration source. This allows the distance calculation unit 74 tocalculate the distance Z from the read reference surface 24 a of theimage reading unit 24 to the noise generation source.

After thus calculating the distance Z from the read reference surface 24a to the noise generation source, the distance calculation unit 74stores, in the storage unit 58, the calculated distance Z in associationwith the noise (Step S42). As a result, when, e.g., six noises areextracted by the noise extraction processing, as illustrated in FIG. 17, six distances (mm) are stored in the storage unit 58 in associationwith the individual noises. In FIG. 17 , the six noises are numberedwith 1 to 6. The respective distances Z corresponding to the first,second, third, fourth, fifth, and sixth noises are 10.5 mm, 10 mm, 10.5mm, 10.7 mm, 10.5 mm, and 10.7 mm, respectively.

FIG. 18 is a diagram illustrating dimensional relations among individualportions of the image reading unit 24 in the image formation device 10according to the embodiment of the present invention.

As illustrated in FIG. 18 , the camera-to-camera distance B is 10 mm,the focal point distance F is 3 mm, and each of the distances B and F isconstant. By contrast, the distance Z described above varies dependingon the amount of displacement D of the noise resulting from theparallax. Specifically, as the amount of displacement D of the noiseresulting from the parallax is smaller, the distance Z is longer. Whenthe distance from the read reference surface 24 a of the image readingunit 24 to the image formation surface 15 c of the paper 15 is 10.5 mmin terms of machine design of the image formation device 10, it ispossible to determine whether or not a noise is present on the imageformation surface 15 c depending on whether the distance Z calculated bythe distance calculation processing S4 described above is equal to 10.5mm or longer or shorter than 10.5 mm by a predetermined amount.

Specifically, when the distance (hereinafter referred to also as the“calculated distance”) calculated by the distance calculation processingS4 is equal to 10.5 mm, it can be determined that a noise is present onthe image formation surface 15 c. When the calculated distance Z isshorter than 10.5 mm by a predetermined amount, it can be determinedthat a noise is present on a front side of the image formation surface15 c. When the calculated distance Z is longer than 10.5 mm by apredetermined amount, it can be determined that a noise is present on arear side of the image formation surface 15 c. A description will begiven below of the noise determination processing based on such adetermination criterion. The predetermined amount is determined by aposition of the heat transfer prevention sheet 27 or a position of thereading plate 66. This is because the foreign substance 80 adhering tothe reading plate 66 or the surface pattern of the heat transferprevention sheet 27 may serve as the noise generation source.

(Noise Determination Processing)

The noise determination processing S5 is processing to be performed bythe noise determination unit 75.

FIG. 19 is a flow chart illustrating the procedure of the noisedetermination processing S5.

First, the noise determination unit 75 acquires a threshold table storedin the storage unit 58 in association with the thickness of the paper 15by reading the threshold table from the storage unit 58 (Step S51). Thethickness of the paper 15 may be detected appropriately by providing theimage formation unit 12 with a paper thickness detection unit 18, asillustrated in FIG. 2 , and by using the paper thickness detection unit18. As a configuration of the paper thickness detection unit 18, aconfiguration can be used in which, e.g., the paper 15 is interposedbetween a pair of thickness detection rollers including a fixed rollerand a movable roller, and an amount of displacement of the movableroller at that time is detected with a sensor or the like.

FIG. 20 is a diagram illustrating an example of the threshold table.

The threshold table illustrated in FIG. 20 roughly includes threethresholds. The first threshold is a threshold for determining whetheror not a noise is present on the front side of the image formationsurface 15 c of the paper 15. The first threshold is set to “10.3 mm orless”. The second threshold is a threshold for determining whether ornot a noise is present on the image formation surface 15 c of the paper15. The second threshold is set to “10.5±0.1 mm”. The third threshold isa threshold for determining whether or not a noise is present on therear side of the image formation surface 15 c of the paper 15. The thirdthreshold is set to “10.7 mm or more”. Each of the thresholds definesthe distance from the read reference surface 24 a to the noisegeneration source.

The second threshold may be set to THRESHOLD=10.5 mm when the distancefrom the read reference surface 24 a to the image formation surface 15 cis 10.5 mm in terms of mechanical design of the image reading unit 24,or may also be set to THRESHOLD=10.5±0.1 mm by taking a marginconsidering a predetermined error into account. The predetermined errorincludes a dimensional error of each component of the image reading unit24 or the like. The threshold table illustrated in FIG. 20 shows anexample in which the margin is set to 0.1 mm by way of example, and asecond threshold is set by taking the margin into account.

The threshold table described above is set such that the position atwhich a noise is present is categorized depending on the case where thedistance from the read reference surface 24 a to the noise generationsource is not less than 10.4 and not more than 10.6 mm (10.5±0.1 mm),the case where the distance is not more than 10.3 mm, or the case wherethe distance is not less than 10.7 mm. Specifically, the threshold tableis set such that, when the distance from the read reference surface 24 ato the noise generation source is not less than 10.4 mm and not morethan 10.6 mm, the noise is categorized as one present on the imageformation surface 15 c of the paper 15, when the distance is not morethan 10.3 mm, the noise is categorized as one present on the front sideof the image formation surface 15 c of the paper 15, and when thedistance is not less than 10.7 mm, the noise is categorized as onepresent on the rear side of the image formation surface 15 c of thepaper 15.

Note that, when the thickness of the paper 15 detected by the paperthickness detection unit 18 is larger, the position of the imageformation surface 15 c of the paper 15 is closer to the read referencesurface 24 a of the image reading unit 24. Meanwhile, when the thicknessof the paper 15 detected by the paper thickness detection unit 18 issmaller, the position of the image formation surface 15 c of the paper15 is more distant from the read reference surface 24 a of the imagereading unit 24. Accordingly, in the storage unit 58, a plurality of thethreshold tables are stored in advance in association with the variousthicknesses of the paper 15. Then, the noise determination unit 75acquires the threshold table by reading, from the storage unit 58, thethreshold table stored in the storage unit 58 in association with thethickness of the paper 15 detected by the paper thickness detection unit18. The acquisition of the threshold table read from the storage unit 58by the noise determination unit 75 corresponds to setting thresholds fordetermining whether or not a noise is present on the image formationsurface 15 c.

Then, by the distance calculation processing S4 described above, thenoise determination unit 75 acquires the calculated distance Z byreading, from the storage unit 58, the calculated distance Z stored inthe storage unit 58 in association with the noise (Step S52).

Then, the noise determination unit 75 compares the calculated distance Zacquired in Step S52 to the thresholds set to the threshold table (StepS53) and categorizes the position at which the noise is present based ona result of the comparison. For example, in the case of using thethreshold table illustrated in FIG. 20 , in the noise determination unit75, YES is given as an answer in Step S54 when the calculated distance Zis not more than 10.3 mm and then categorizes the noise as one presenton the front side of the image formation surface 15 c of the paper 15(Step S55). Meanwhile, when the calculated distance Z exceeds 10.3 mm,in the noise determination unit 75, NO is given as an answer in Step S54and proceeds to processing in Step S56.

Then, the noise determination unit 75 gives YES as an answer in Step S56when the calculated distance Z is not less than 10.4 mm and not morethan 10.6 mm, and then categorizes the noise as one present on the imageformation surface 15 c of the paper 15 (Step S57). Meanwhile, when thecalculated distance Z exceeds 10.6 mm, the noise determination unit 75gives NO as an answer in Step S56, proceeds to processing in Step S58,and categorizes the noise as one present on the rear side of the imageformation surface 15 c of the paper 15 (Step S58).

In such categorization processing, it may also be possible to allow anotification unit to notify the user whether or not a noise is presenton the image formation surface 15 c. The notification unit can be formedof, e.g., the operation/display unit 53. When the notification unit isformed of the operation/display unit 53, a message telling whether ornot a noise is present on the image formation surface 15 c may bedisplayed appropriately on the operation/display unit 53.

In the categorization processing described above, when a noise iscategorized as one present on the front side of the image formationsurface 15 c of the paper 15, it is highly possible that a cause of thenoise is the foreign material 80 adhering to the reading plate 66.Accordingly, the noise determination unit 75 performs, as processingafter the processing in Step S55 is performed, reporting processing ofencouraging the user to perform a predetermined maintenance operation(Step S60). The predetermined maintenance operation includes anoperation of checking whether or not the foreign substance 80 adheres tothe reading plate 66 of the image reading unit 24 and an operation ofremoving the foreign substance 80 adhering to the reading plate 66.Examples of the reporting processing include processing of displaying amessage on the operation/display unit 53, processing of transmitting themessage from the communication unit 59 to a terminal device held by theuser to cause a display screen of the terminal device to display themessage, and the like. Alternatively, the reporting processing may alsobe processing of outputting a sound.

When a noise is categorized as one present on the rear side of the imageformation surface 15 c of the paper 15, a conceivable cause of the noiseis unexpected imaging of the conveyance drum 20 or the heat transferprevention sheet 27 appearing as the noise. In that case, the noisedetermination processing S5 is immediately ended.

Meanwhile, when a noise is categorized as one present on the imageformation surface 15 c of the paper 15, the noise determination unit 75specifies a cause of the noise as the missing nozzle defect 17 (StepS61). Then, the noise determination unit 75 sets the position of thenoise the cause of which is specified as the missing nozzle defect 17 toa target position to be subjected to correction by the image correctionunit 76 (Step S62).

Note that the noise determination unit 75 may also have a thresholdsetting function as described below. The threshold setting function is afunction of setting the thresholds based on the first image 81 a and thesecond image 81 b which are obtained through reading of the image on thepaper 15 including the missing noise defect 17 by the first read sensor62 a and the second read sensor 62 b. Specifically, before theprocessing illustrated in FIG. 10 is started, the image on the paper 15including the missing nozzle defect 17 is read by the first read sensor62 a and the second read sensor 62 b. At this time, each of the firstimage 81 a obtained by the first read sensor 62 a and the second image81 b obtained by the second read sensor 62 b includes the noise due tothe missing nozzle defect 17. The position of the noise on the firstimage 81 a and the position of the noise on the second image 81 b aredisplaced from each other in the main scanning direction. Accordingly,the noise determination unit 75 causes the displacement detection unit73 to detect an amount of displacement between the noise on the firstimage 81 a and the noise on the second image 81 b and gives the amountof displacement as a result of the detection to the distance calculationunit 74 to cause the distance calculation unit 74 to calculate thedistance from the read reference surface 24 a to the noise generationsource. Then, the noise determination unit 75 sets the distance as aresult of the calculation by the distance calculation unit 74 or a valueobtained by adding a margin to the distance as the second threshold. Inaddition, the noise determination unit 75 sets the first threshold to avalue less than the second threshold and also sets the third thresholdto a value exceeding the second threshold. By providing the noisedetermination unit 75 with such a threshold setting function, even whenthe thickness of the paper 15 cannot be specified, it is possible toappropriately set the thresholds based on the thickness of the paper 15.

The noise determination unit 75 having the threshold setting functionmay also be configured to set the thresholds based on the first image 81a and the second image 81 b each obtained by the first read sensor 62 aand the second read sensor 62 b through reading of the image on thepaper 15 including a pattern which allows the parallax to be specified(hereinafter referred to as the “parallax specification pattern”),instead of the image 16 on the paper 15 including the missing nozzledefect 17 described above. In this configuration, each of the firstimage 81 a obtained by the first read sensor 62 a and the second image81 b obtained by the second read sensor 62 b includes a noise due to theparallax specification pattern. Accordingly, the noise determinationunit 75 can set the threshold by the same method as described above. Theparallax specification pattern may be any pattern as long as the patternallows the parallax to be specified. By way of example, as the parallaxspecification pattern, a linear pattern parallel with the sub-scanningdirection Y can be used.

(Image Correction Processing)

The image correction processing is processing to be performed by theimage correction unit 76. The image correction unit 76 performs theimage correction processing only when it is determined by the noisedetermination processing S5 described above that a noise is present onthe image formation surface 15 c. The image correction processing isprocessing of correcting a missing portion of the image due to themissing nozzle defect 17. The image correction unit 76 performs theimage correction processing by the noise determination processing S5described above on the assumption that a position of a noise specifiedas a noise due to the missing nozzle defect 17 is a correction targetposition. The position of the noise due to the missing nozzle defect 17corresponds to a position of the one of the nozzles 244 in which an inkejection failure has occurred when an image is formed on the paper 15.

Accordingly, in the image correction process, the image correction unit76 first specifies, based on the position of the noise due to themissing nozzle defect 17, the position of the nozzle 244 in which theink ejection failure has occurred. In the present embodiment, by way ofexample, it is assumed that, the ink ejection failure has occurred in anozzle 244 a among the plurality of nozzles 244 illustrated in FIG. 4 .

Then, the image correction unit 76 changes conditions under which thehead drive unit 55 drives the ink-jet head 32 such that the nozzle 244adjacent to the nozzle 244 a corresponding to a faulty nozzle or thenozzle 244 serving as a substitute for the nozzle 244 a compensates foran image to be formed by the nozzle 244 a. As a result, when the imageis formed on the paper 15 conveyed after the paper 15 with the missingnozzle defect 17, it is possible to eliminate or reduce the occurrenceof the missing nozzle defect 17 due to the faulty nozzle. Therefore, itis possible to prevent degradation of an image quality resulting fromthe ink ejection failure during a period until the nozzle ejectionfailure is eliminated.

(Image Quality Adjustment Processing)

Image quality adjustment processing is processing to be performed by theimage quality adjustment unit 77.

The image quality adjustment unit 77 performs image quality adjustmentbased on the first image 81 a obtained by the first read sensor 62 aand/or on the second image 81 b obtained by the second read sensor 62 b.At that time, as illustrated in FIG. 8 , when the foreign substance 80adheres to the reading plate 66, the foreign substance 80 causes a noisein each of the first image 81 a and the second image 81 b. The noiseappears as a partially missing portion of the image. Consequently, whenthe image quality adjustment unit 77 performs the image qualityadjustment based on the first image 81 a, information required for theimage quality adjustment cannot be obtained from a portion of the firstimage 81 a in which the noise is observed.

Accordingly, the image quality adjustment unit 77 according to thepresent embodiment compensates for the portion with the noise present inone of the first image 81 a and the second image 81 b with a portion ofthe other image corresponding to a position of the noise to perform theimage quality adjustment. When the image formation surface 15 c of thepaper 15 is to be read with the image reading unit 24, the first readsensor 62 a and the second read sensor 62 b simultaneously image theimage formation surface 15 c of the paper 15 in different directions. Asa result, when the foreign substance 80 adheres to the reading plate 66as illustrated in FIG. 8 , the position of the noise appearing in thefirst image 81 a, while being blocked by the foreign substance 80, isdifferent from the position of the noise appearing in the second image81 b, while being blocked by the foreign substance 80. In other words,the portion of the image blocked by the foreign substance 80 andinvisible to the first read sensor 62 a is visible to the second readsensor 62 b without being blocked by the foreign substance 80. Likewise,the portion of the image blocked by the foreign substance 80 andinvisible to the second read sensor 62 b is visible to the first readsensor 62 a without being blocked by the foreign substance 80. In otherwords, a missing portion of one of the first image 81 a and the secondimage 81 b which corresponds to a noise is viewed as a normal portionwith no missing part in the other image.

Accordingly, when the image quality adjustment is performed based one.g., the first image 81 a, even though the foreign substance 80 causesa noise in the first image 81 a, the image adjustment unit 77compensates for the portion with the noise, i.e., the missing portion ofthe image with a portion of the second image 81 b corresponding to aposition of the noise. This allows the image quality adjustment unit 77to perform the image quality adjustment based on the image with nomissing portion. As a result, it is possible to implement excellentimage quality adjustment highly resistant to a noise.

Effects of Embodiment

As described above, in the embodiment of the present invention, theamount of noise displacement resulting from the parallax between thefirst read sensor 62 a and the second read sensor 62 b is determinedbased on the first image 81 a obtained by the first read sensor 62 a andon the second image 81 b obtained by the second read sensor 62 b and,based on the determined amount of displacement, it is determined whetheror not a noise is present on the image formation surface 15 c. Thisallows a noise due to the missing nozzle defect 17 to bediscriminatively recognized from the other noise. As a result, when theimage 16 on the paper 15 is to be read with the image reading unit 24,it is possible to more reliably determine whether or not the noiseincluded in each of the images 81 a and 81 b obtained as a result of thereading is a noise due to the ink ejection failure in the nozzle.

Modifications

The technical scope of the present invention is by no means limited bythe embodiment described above, and includes even a mode obtained byadding various changes and improvements to the embodiment describedabove in a scope which allows specific effects obtainable withconstituent features of the invention or a combination thereof to bederived.

For example, the image formation device 10 may also have a configurationwhich includes a paper determination unit that determines whether or notthe paper 15 to be subjected to reading by the image reading unit 24 hasa light transmissivity and in which, when the paper 15 has no lighttransmissivity in a result of the determination by the paperdetermination unit, the distance calculation unit 74 calculates thedistance Z within a range of not more than a threshold defining thedistance from the read reference surface 24 a to the image formationsurface 15 and, when the paper 15 has a light transmissivity, thedistance calculation unit 74 calculates the distance Z within a range ofnot more than the foregoing threshold and exceeding the foregoingthreshold. By using such a configuration, it is possible to reduce aprocessing load placed on arithmetic operations performed by thedistance calculation unit 74.

Note that the threshold defining the distance from the read referencesurface 24 a to the image formation surface 15 c refers to the secondthreshold described above. The paper determination unit may also beconfigured to include, e.g., a transmission-type photosensor having alight emitting unit and a light receiving unit facing each other via thepaper conveyance path, allow the light receiving unit to receive lightemitted from the light emitting unit and transmitted by the paper 15,and determine whether or not the paper 15 has a light transmissivitybased on an amount of the received light. Alternatively, the paperdetermination unit may also be configured to determine whether or notthe paper 15 has a light transmissivity based on information related tothe paper 15 that has been input by the user by operating theoperation/display unit 53.

Alternatively, the image formation device 10 may also have aconfiguration which includes a paper size detection unit that detects asize of the paper 15 and in which the noise extraction unit 71determines a target region from which a noise is to be extracted basedon the size of the paper 15 detected by the paper size detection unit.By using such a configuration, when the size of the paper 15 is large,it is possible to ensure the larger-sized target region from which anoise is to be extracted while, when the size of the paper 15 is small,it is possible to provide the smaller-sized target region from which anoise is to be extracted. Therefore, it is possible to efficientlyperform the noise extraction processing, while reliably extracting thenoise included in the first image 81 a.

Note that the paper size detection unit may also have a configurationwhich includes, e.g., a linear image sensor disposed to face a directionperpendicular to a widthwise end portion of the paper 15 passing throughthe paper conveyance path and in which the size of the paper 15 in apaper width direction is detected based on a position of the widthwiseend portion of the paper 15 which is detected by the linear imagesensor. Alternatively, the paper size detection unit may also beconfigured to detect the size of the paper 15 based on informationrelated to the paper 15 that has been input by the user by operating theoperation/display unit 53.

In the embodiment described above, the read sensor 62 a as one of thetwo read sensors 62 a and 62 b provided in the image reading unit 24 isused as the first read sensor and the read sensor 62 b as the other readsensor is used as the second read sensor, but the present invention isnot limited thereto. In other words, it may also be possible to use theread sensor 62 b as the first read sensor and use the read sensor 62 aas the second read sensor.

In the embodiment described above, the conveyance drum 20 has beendescribed as an example of the sheet conveyer, but the present inventionis not limited thereto. The present invention is also applicable to asheet conveyer including a conveyance belt.

Also, in the embodiment described above, a description has been given ofthe case where the noise determination unit 75 has the threshold settingfunction, but the present invention is not limited thereto. It may alsobe possible that a threshold setting unit is provided separately fromthe noise determination unit 75, and the noise determination unit 75determines, based on the thresholds set by the threshold setting unit,whether or not a noise is present on the image formation surface.

Also, in the example shown in the embodiment described above, the imageformation unit 12 is provided with the paper thickness detection unit18, but the present invention is not limited thereto. For example, whenthe user operates the operation/display unit 53 to input informationabout the paper 15 and the thickness of the paper 15 can be detectedfrom the input information, the operation/display unit 53 is allowed tofunction as the paper thickness detection unit. As the information aboutthe paper 15 input by the user, information such as a type or brand ofthe paper 15 can be considered.

Each of the displacement detection processing, the distance calculationprocessing, and the noise determination processing is performed for eachnoise extracted by the noise extraction processing. When a plurality ofnoises are extracted by the noise extraction processing, it may bepossible to sequentially perform the displacement detection processing,the distance calculation processing, and the noise determinationprocessing for the plurality of noises. Alternatively, it may also bepossible to perform the displacement detection processing, the distancecalculation processing, and the noise determination processing for oneof the noises, and then perform the displacement detection processing,the distance calculation processing, and the noise determinationprocessing for each of the other noises in the same manner as for thepreceding noise.

The present invention can be implemented not only as the image formationdevice, but also an image reading device, an image reading method, or anon-transitory recording medium storing a computer readable program, andcan also be implemented as an image forming method. When the presentinvention is implemented as the image reading device, the image readingdevice is configured to include at least the image reading unit 24 andthe control unit 51.

While the present specification has used terms such as “parallel” and“perpendicular”, each of the terms does not mean only “parallel” or“perpendicular” in a strict sense, but also means, in addition to“parallel” or “perpendicular” in a strict sense, “substantiallyparallel” or “substantially perpendicular” within a range where thefunctions can be exhibited.

Although embodiments of the present invention have been described andillustrated in detail, the disclosed embodiments are made for purposesof illustration and example only and not limitation. The scope of thepresent invention should be interpreted by terms of the appended claims.

REFERENCE SIGNS LIST

-   10 Image formation device-   15 Paper (Sheet)-   15 c Image formation surface-   17 Missing nozzle defect-   18 Paper thickness detection unit (Sheet thickness detection unit)-   20 Conveyance drum (Sheet conveyer)-   24 Image reading unit-   24 a Read reference surface-   51 Control unit-   53 Operation/display unit (Notification unit)-   62 Read sensor-   62 a First read sensor-   62 b Second read sensor-   64 Reducing optical system-   64 a First reducing optical system-   64 b Second reducing optical system-   71 Noise extraction unit-   73 Displacement detection unit-   74 Distance calculation unit-   75 Noise determination unit-   76 Image correction unit-   77 Image quality adjustment unit-   81 a First image-   81 b Second image-   82, 83, 84 Noise-   X Main scanning direction

What is claimed is:
 1. An image reading device comprising: an imagereading unit that reads an image formation surface of a sheet by using areducing optical system; and a control unit, wherein the image readingunit includes a plurality of read sensors disposed at positionsdisplaced from each other in a main scanning direction, the plurality ofread sensors are disposed to be able to read the image formation surfaceby using at least a local region of the image formation surface as acommon region to be read, and the control unit determines, based on afirst image obtained by a first read sensor included in the plurality ofread sensors and on a second image obtained by a second read sensorincluded in the plurality of read sensors and different from the firstread sensor, an amount of displacement of a noise resulting from aparallax between the first read sensor and the second read sensor anddetermines, based on the amount of displacement, whether or not thenoise is present on the image formation surface.
 2. The image readingdevice according to claim 1, wherein the control unit includes: a noiseextraction unit that extracts the noise from the first image; adisplacement detection unit that specifies a position on the secondimage at which the noise extracted by the noise extraction unit ispresent and detects, based on information about the specified position,an amount of displacement corresponding to an amount of displacementbetween a position on the first image at which the noise is present andthe position on the second image at which the noise is present; adistance calculation unit that calculates a distance from a readreference surface of the image reading unit to a generation source ofthe noise based on the amount of displacement detected by thedisplacement detection unit; and a noise determination unit thatdetermines whether or not the noise is present on the image formationsurface based on the distance calculated by the distance calculationunit.
 3. The image reading device according to claim 2, wherein thenoise determination unit sets a threshold for determining whether or notthe noise is present on the image formation surface.
 4. The imagereading device according to claim 3, wherein the noise determinationunit sets the threshold based on the first image and the second imagewhich are obtained through reading of an image on the sheet including amissing nozzle defect by the first read sensor and the second readsensor.
 5. The image reading device according to claim 3, furthercomprising: a sheet thickness detection unit that detects a thickness ofthe sheet, wherein the noise determination unit sets the threshold basedon the thickness of the sheet detected by the sheet thickness detectionunit.
 6. The image reading device according to claim 3, wherein thenoise determination unit sets the threshold based on the first image andthe second image which are obtained by the first read sensor and thesecond read sensor through reading of an image on the sheet including apattern that allows the parallax to be specified.
 7. The image readingdevice according to claim 3, further comprising: a sheet determinationunit that determines whether or not the sheet to be subjected to thereading by the image reading unit has a light transmissivity, wherein,when the sheet has no light transmissivity in a determination resultfrom the sheet determination unit, the distance calculation unitcalculates a distance from the read reference surface to the imageformation surface within a range of not more than a threshold definingthe distance and, when the sheet has a light transmissivity in thedetermination result from the sheet determination unit, the distancecalculation unit calculates the distance within a range of not more thanthe threshold and exceeding the threshold.
 8. The image reading deviceaccording to claim 3, wherein the noise determination unit determineswhether or not the noise is present on the image formation surface basedon a result of a comparison between the distance calculated by thedistance calculation unit and the threshold.
 9. The image reading deviceaccording to claim 2, further comprising: a sheet size detection unitthat detects a size of the sheet, wherein the noise extraction unitdetermines a region from which the noise is to be extracted depending onthe size of the sheet detected by the sheet size detection unit.
 10. Theimage reading device according to claim 2, wherein the displacementdetection unit specifies the position on the second image at which thenoise extracted by the noise extraction unit is present by imagematching processing.
 11. The image reading device according to claim 10,wherein the displacement detection unit performs the image matchingprocessing on a region set in advance.
 12. The image reading deviceaccording to claim 10, wherein the noise extraction unit determines,when a plurality of the noises are extracted from the first image, aregion including the plurality of noises to be an image matching region,and the displacement detection unit specifies a position on the secondimage at which the image has a highest degree of matching with an imagein the image matching region as a position at which the plurality ofnoises are present.
 13. The image reading device according to claim 2,wherein the noise extraction unit detects a linear noise as the noise.14. The image reading device according to claim 2, wherein the noiseextraction unit extracts at least any one of a noise due to mist, dust,or paper powder present between the image formation surface of the sheetand the read reference surface of the image reading unit, a noise due toa missing nozzle defect appearing on the image formation surface of thesheet, a noise due to a sheet conveyer that conveys the sheet, and anoise due to a heat transfer prevention sheet covering a surface of thesheet conveyer.
 15. The image reading device according to claim 2,wherein the noise determination unit performs reporting processing ofencouraging a user to perform a predetermined maintenance operation whendetermining that the noise is present on a front side of the imageformation surface when viewed from the read sensor.
 16. The imagereading device according to claim 1, further comprising: a notificationunit that notifies a user whether or not the noise is present on theimage formation surface.
 17. An image reading method using an imagereading device including an image reading unit that reads an imageformation surface of a sheet by using a reducing optical system andincludes a plurality of read sensors disposed at positions displacedfrom each other in a main scanning direction, the plurality of readsensors being disposed to be able to read the image formation surface byusing at least a local region of the image formation surface as a commonregion to be read, the image reading method comprising: determining,based on a first image obtained by a first read sensor included in theplurality of read sensors and on a second image obtained by a secondread sensor included in the plurality of read sensors and different fromthe first read sensor, an amount of displacement of a noise resultingfrom a parallax between the first read sensor and the second readsensor; and determining, based on the amount of displacement, whether ornot the noise is present on the image formation surface.
 18. Anon-transitory recording medium storing a computer readable program forcausing a computer of an image reading device including an image readingunit that reads an image formation surface of a sheet by using areducing optical system and includes a plurality of read sensorsdisposed at positions displaced from each other in a main scanningdirection, the plurality of read sensors being disposed to be able toread the image formation surface by using at least a local region of theimage formation surface as a common region to be read, to execute:determining, based on a first image obtained by a first read sensorincluded in the plurality of read sensors and on a second image obtainedby a second read sensor included in the plurality of read sensors anddifferent from the first read sensor, an amount of displacement of anoise resulting from a parallax between the first read sensor and thesecond read sensor; and determining, based on the amount ofdisplacement, whether or not the noise is present on the image formationsurface.
 19. An image formation device that causes each of a pluralityof nozzles to eject ink to form an image on a sheet, the image formationdevice comprising: an image reading unit that reads an image formationsurface of the sheet by using a reducing optical system; and a controlunit, wherein the image reading unit includes a plurality of readsensors disposed at positions displaced from each other in a mainscanning direction, the plurality of read sensors are disposed to beable to read the image formation surface by using at least a localregion of the image formation surface as a common region to be read, andthe control unit determines, based on a first image obtained by a firstread sensor included in the plurality of read sensors and on a secondimage obtained by a second read sensor included in the plurality of readsensors and different from the first read sensor, an amount ofdisplacement of a noise resulting from a parallax between the first readsensor and the second read sensor and determines, based on the amount ofdisplacement, whether or not the noise is present on the image formationsurface.
 20. The image formation device according to claim 19, furthercomprising: an image correction unit that corrects a missing portion ofan image due to a missing nozzle defect, wherein the image correctionunit performs the correction only when it is determined that the noiseis present on the image formation surface.
 21. The image formationdevice according to claim 19, further comprising: an image qualityadjustment unit that adjusts a quality of the image formed on the sheet,wherein the image quality adjustment unit compensates for a portion of anoise observed in one of the first image and the second image with aportion of the other image corresponding to a position of the noise, toperform the image quality adjustment.